This invention relates to a driving circuit employing insulated-gate field-effect transistors and more particularly, to a driving circuit of a boot-strap type which produces an output signal having a value of a power source voltage level by utilizing a boot strap effect.
A boot-strap circuit utilizing a boot-strap effect has gained a wide application as a driving circuit which can produce at an output terminal an output signal of a level equal to the power source voltage level as one of the two logic levels and has a large load driving capacity. For the boot-strap effect, a capacitor is connected between the output terminal and a gate of a first field effect transistor, which is in turn coupled between a power source terminal and the output terminal.
A second field effect transistor is coupled between the output terminal and a ground terminal and an input signal is applied to the gates of the first and the second transistors in the opposite phase and in the same phase, respectively. A delayed input signal is applied, in phase, to a gate of a third transistor which is coupled between the output terminal and the ground terminal. When the input signal changes from a high level to a low level, the second transistor turns to the non-conducting state, but the output terminal is clamped at a ground potential by the third transistor whose gate is still high level. Therefore, the boot-strap capacitor is changed by the high level applied to the gate of the first transistor. After a delay time is elapsed, the third transistor turns to the non-conducting state and a potential at the output terminal starts to rise toward the power source voltage which is applied to the output terminal through the conducting first transistor.
The rise of the potential at the output terminal is superposed on the gate potential of the first driving transistor through the charged capacitor. Accordingly, the gate potential of the first transistor rises above the power source level and therefore, this first transistor or a driving field effect transistor connected to the gate of the first transistor is driven in a non-saturation region to output the power source voltage without level reduction.
A typical example of the boot-strap circuit of the kind described above is disclosed in U.S. Pat. No. 3,898,479. However, the conventional boot-strap circuit of the prior art involves some drawbacks. After the boot-strap capacitor is changed and the third transistor is turned to the non-conducting state in response to the low level of the input signal, if a high level noise signal is added to the input signal, the second transistor becomes conducting to discharge the boot-strap capacitor, and the boot-strap effect is no longer exhibited even if the input signal returns thereafter to the low level. As a result, the potential at the output terminal cannot be raised higher than a value which is lower than the power source level by the threshold voltage of the field effect transistor. If the input signal takes the high level for a shorter period of time than the delay time for the gate signal of the third transistor, the output terminal is not clamped at the ground potential for the following low level input signal and the boot-strap capacitor is not charged to lose the boot-strap function.